1. Field of the Invention
The present invention relates to bound microcapsules used to deliver pigments and other colorant materials and to articles containing such microcapsules.
2. Background of the Art
It is fairly common to find encapsulated liquid materials in the marketplace. Technology has been available for many years to effectively provide microcapsules containing liquid oleophilic ingredients. Representative processes are shown in U.S. Pat. Nos. 3,016,308 and 3,516,941. These patents disclose in situ polymerization reactions in which a hydrophobic oil phase is dispersed in an aqueous phase containing resin precursors, particularly aminoplast resin precursors (to form urea/aldehyde resins and the like). High shear agitation is used to keep the capsule size small. Addition of an acid catalyst initiates the polycondensation of the aminoplast precursors, resulting in the deposition of the aminoplast resin about the dispersed droplets of the oil phase. This produces the microcapsules.
Other polycondensation encapsulation techniques are shown in U.S. Pat. Nos. 3,429,827 and 4,000,087. These particular techniques are more limited in the classes of hydrophobic inner phases acceptable in the microcapsules because of reaction with the oil soluble monomer or poor solubility of the monomer in the desired hydrophobic phase.
U.S. Pat. No. 3,930,101 teaches that, to be retained in the hydrophobic phase during high shear dispersion of a fluid particulate dispersion, it is necessary that the particulate be preferentially wetted by the hyrophobic phase. It is suggested to use suitable surfactants which adsorb to the particulate surface as a way to achieve the desired preferential wetting. It has, however, been recognized that, in the in situ polymerization of aminoplast resins method for encapsulation, the presence of surfactants interferes with the deposition of the aminoplast resin at the hydrophobic phase/water phase interface, giving poorly formed or leaky capsules. Similarly, oil soluble suspending agents could alter the wetting of many particulates. Since many of these materials contain carboxylate groups, exposure to highly acidic medias often converts them to carboxylic acid groups altering their adsorbability to the particulates.
U.S. Pat. No. 4,307,169 teaches the inclusion of magnetic materials into a pressure fixable core material within a shell formed by interfacial polycondensation. It is also stated that a dye may be included in the aqueous phase during encapsulation so that it is deposited on the outside of the capsular shell and bonded thereto.
U.S. Pat. No. 3,954,666 teaches the preparation of semipermeable microcapsules containing catalysts and ferromagnetic materials.
U.S. Pat. No. 4,450,221 teaches magnetic toners comprising lyophilic magnetic particles and a resin surrounded by a resin wall to form microcapsules. Colorants such as pigments or dyes may be included in the wall forming resin or the toner. The magnetic particles are rendered lyophilic by treatment with a titanate or silane coupling agent. The coupling agent is said to uniformly disperse the particles in the binder resin and firmly bond the magnetic particle to the resin.